


Pranks Gone Wrong

by Nillas NSFW Corner (Agraulis_vanillae)



Category: Undertale (Video Game)
Genre: Arachnophobia, Explicit Sexual Content, Height difference, M/M, Prank Wars, Pranks and Practical Jokes, Sexual Content, Sexual Tension, Sexually Frustrated Swap, Shameless Smut, Smut, Socketfucking, Swap has 'Regrets', Tentacles, honeymustard - Freeform
Language: English
Status: Completed
Published: 2017-01-05
Updated: 2017-01-05
Packaged: 2018-09-15 01:20:58
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,370
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/9213077
Author URL: https://archiveofourown.org/users/Agraulis_vanillae/pseuds/Nillas%20NSFW%20Corner
Summary: Red and Swap don't get along at all, but that's fine. It's only a prank war after all.





	

**Author's Note:**

  * For [Leedlepie](https://archiveofourown.org/gifts?recipient=Leedlepie).



> This was written for Leedlepie for Secret Santa today! I really hope I wrote this how you'd like it, Leeds!

During their first meeting in Swap’s universe, he and Red both made the mistake that the other would be like their brother. Red held his hand out on introduction, and Swap took it.

 

“nice to meet you, ‘bro’.”

 

In that instant, Swap was zapped with a mild shock from a joy buzzer and Red got whipped cream sprayed straight into his face. Needless to say, neither of them took being caught off guard particularly well, with Red scowling through the whip cream and Swap grimacing at the ghost sensation of electricity lacing through his palm.With both of them shaking off the aftereffects of the pranks, it became war from day one.

 

Inexplicably and in stark contrast to themselves, their brothers got along like a wildfire. The grumpy standoffish Fell seemed to regard the hyperactive optimistic Blue as some sort of excitable puppy. Fell responded to this by acting like a hyper vigilant guard, keeping Blue from getting hurt in any way possible. This strange alliance meant that Swap couldn't just shove them both back into the alternate universe where they came from, and grudgingly he admitted to himself that he wouldn’t want to. His stress and the number of cigarettes he burned through decreased directly in response, as Fell’s presence relieved him of the strange worried feeling of some shadow looming over his brother...

 

That didn't mean that Swap wouldn't regret allowing them both to stay, as he was pressed to his japing limits to stay on top of Red's tricks. One time Red threw a can of air freshener with a zip tie to keep the toggle depressed while Swap was gone. After he finished tending a stand where he served sopapillas, sneaking spoonfuls of honey that was available to serve to the customers, he came home to his room smelling overwhelmingly of artificial lemon scent. He remembered to keep his room locked after that.

 

In return, he set up an air horn to go off when Red sat down on the couch. Red didn't visibly check the seats, but he never fell for it again afterwards.

 

Other things Swap learned pretty quickly about was not to exit from the door of his bedroom, or anywhere in the immediate vicinity of it given there was a wire holding up a bucket of lukewarm water attached to the door handle. The kitchen was safe, and so was Blue's room, but anywhere outside was fair game. Truthfully, anywhere that was out of their brother's sight was fair game. At first the trend for Red was to leave piles of joke vomit all over the place, only effective if Swap made the mistake of looking down, which happened more often than he'd like to his own consternation. 

 

“SANSSS!”

 

Fortunately, this only kept up until Fell came back home and threw a hissy fit, a phrase no more apt than now when the air whistling past Fell's teeth as he gritted them sounded eerily similar to a pissed off cat. It seems that his surly counterpart shared his distaste for anything puke related and made it VERY clear that it wasn't allowed to be in the house. Or in vicinity of the house. Or in Snowdin for that matter. That territory was probably going to spread as the sticky notes continued to travel down the wall in their room in the old lab behind the house.

 

In return, Swap would tamper with the bottles of mustard that had to be refrigerated unlike honey which he could keep in his locked room. Now, Red usually drank mustard by uncapping the lid first. Another prank Swap pulled in return was placing toy spiders in unexpected places like the cabinets, but he found out real quick that was that spiders gave both Red and Fell panic attacks, and so he stopped after the first time he saw Red back off with sweat pouring from his skull and wide unseeing eyes. 

 

Their pranks remained ineffective and frankly pretty wimpy even by Blue's standards for a while after that, both monsters afraid of doing something that would actually cause damage to the other and place their HP at risk. It was only supposed to be a series of running gags and one-upping the other, but emotions were a tricky thing, and intent was not always a conscious choice. Neither of them were willing to risk damaging the 1 HP the other had.

 

That changed one day when Red accidentally startled Swap and got elbowed hard in the face for his trouble. Swap pulled his hit as he saw who it was, but turned and grabbed Red's face with the certainty his brother's jaded counterpart would crumble to dust any moment.

 

“ow, the fuck was that for?!” Red swatted away Swap's hands with an irritated huff.

 

“my bad, it was an accident...”Swap muttered as he was staring into Red's face searchingly, leaning in to examine him closely. Even though it seemed that Red's cheekbone was a bit bruised, his soul had taken no damage whatsoever. The two of them stopped to consider what had just occurred. Maybe it was because he looked so much like Blue, but even when startled, Swap couldn’t muster the ill-intent to damage Red.

 

And then Red punched him in the face.

 

“ah fu-! look i'm sorry okay?!” Falling back a bit from the shocking force, Swap grabbed his jaw and rubbed the soreness.

 

“heheh... i didn't punch ya out of vengeance or nothin'.” Red's fixed grin widened ominously. “i did it just 'cuz i can.”

 

Swap smiled, the corners of his mouth twitching, “if you're gonna be like that...”

 

He tackled him, and the next few minutes was spent in a gripping writhing scuffle with both of them trying to hit everything they could get a hold of. They were pulled away minutes later by their brothers-

 

“KNOCK IT OFF, YOU TWO!” 

 

“STOP FIGHTING, OR I’LL GET ALPHYS!!!” 

 

-but the only thing holding them back had already been broken.

 

Their pranks got out of control, escalating to Swap pouring water on the back door porch so it froze over and got slippery so that Red slid and landed hard on his tailbone. Red got even when he planted a Roman candle in Swap's mailbox so he had to snatch the mail in an instant and shortcut out of the way before anything got caught on fire. 

 

Red set up speakers to broadcast scary sounds at night and terrified even Blue, so Swap planted a walkie talkie deep in the mattress that Red slept in a different night and started out breathing heavily into it just to be creepy. It's not like he could sleep, so he could keep it up all night and even if he needed a break, he could just stick his phalanges into his honey bottle and make weird squelching noises with it before licking it off. That was amusing until Red found it, with absolutely no warning until the ‘Talk’ button was pushed.

 

“hey buddy, if you wanted phone sex, why didn't you say so?”

 

If Swap had eyes, they'd be bugging out of his skull at this point, forgetting to let go of his own ‘Talk’ button and slapping a hand over his face. The clacking sound transmitted through the receiver and dark chuckle sounded. “heheh, i'll take that as a yes please.”

 

Before Swap could think of what to do, tensing up and ready to throw the walkie talkie like it was hotter than a two dollar pistol, a drawn out moan sounded from the speaker. He relaxed briefly, and laughed. “nyeh heh heh. that's pretty fake even for you, red.”

 

“do you want it to be real?” Red responded slyly. A resounding “no” was on the tip of Swap's metaphorical tongue, but he caught himself in time. That would only egg him on. Unfortunately, he couldn’t think of a good quip for this particular situation. Red continued speaking regardless, sadistic pleasure leaking through the device, “all right, i'll make it real. lemme grab some tape to nail down that pesky talk button, it'd be a shame if you missed anything, now wouldn't it?”

 

Swap briefly considered placing the walkie talkie outside where anyone passing by could hear Red, and let him chatter himself out to the snow and Gyftmas presents, but despite himself, he was curious.

 

“all right, i'm getting my dick out. i ain't hard yet, after all i'm talking to a skinny asshole but i'm sure i can talk myself 'up' to it, heheheh. i'll place this right here by the action-” some muffled sounds of sheets moving past the other receiver and then it was quiet for a few moments before the muted 'puh' of Red spitting! 

 

“i doubt you can hear a damn thing just yet, but since you've been so eager for a good fuck, i'm rubbing right here at the slit of my dick. ahhh yeah, that's the stuff... hey if i get you to touch yourself -huuuh- that means i win this little game of ours doesn't it? yeahhh heheh let's call it checkmate.” Red was quiet for a moment, and then Swap heard the wet muted sounds, at a rapid pace. It struck Swap as something gimmicky as fisting a cup of noodles or something dumb like him playing with the honey bottle but he remained quiet.

 

“hnnng, f-fuck, you b-better be listening swap. quitting is a white flag, buddy.” Red's groans sounded far off, and the sounds of fapping remained steady. Swap was beginning to feel uncomfortable. He really wasn't jerking off, right? The discomfort increased when he heard an entirely novel noise through the walkie talkie. 

 

Squelch!

 

That certainly wasn't the sound of fingers pumping into a honey bottle. Swap felt his stress rise to an unbearable point of tension, now pacing through the room with a unlit cigarette clamped between his teeth and slowly coming to terms with his growing awareness that Red was actually jacking off on the other side of the house. Not only that, but his previously teasing and mostly put on moans and sighs were increasing not in volume but in intensity and sincerity. The squelching also started to join the symphony of sex and Swap involuntarily pictured Red fucking himself on his fingers as he stroked himself at an increasingly needy pace…

 

“fuh, huhhh gawwwd...” Red drawled breathlessly, the pitch of his voice dipping lower and Swap had to resist the urge to light the cigarette. His brother would kill him if he smelled tobacco in the house. He sat on his bed reluctantly, both tired from being up this late and now fighting with his magic reacting. He closed his eyesockets briefly only to realize that's a terrible idea. His mind was more than happy to supply the picture of Red pumping his cock quickly in the darkened lab, seminal fluid dribbling down the shaft and lubricating the process as he drove his hips down. He’d grind down hard onto his fingers and then thrust back up into his hand, bones glistening dimly with perspiration, mattress creaking through the walkie talkie. 

 

He opened his eyesockets quickly and stared holes into the blank ceiling, trying to imprint the whiteness into his mind instead. Like Red said, this was now part of the competition. He couldn't get off to Red or he'd be well and thoroughly japed by his own prank. Similarly, putting the walkie talkie outside would now be as good as throwing in the towel.

 

He endured the increasing urgency that Red began to groan, now chanting a low mantra, “c'mon, c-come... on- please please please pleasepleaseplease huh-uuuuuuuhhh! ohhh!”

 

The sounds stuttered to a stop and then it was just panting for about a minute, telling him that Red just finished. He heaved a sigh of relief, and then the walkie talkie registered fabric shifting. Red asked smugly:

 

“so, didja touch yourself?”

 

As evenly as Swap could when fighting his dismay at his pants being that much tighter, depressed the talk button on the cursed walkie talkie. “in your dreams, you freak.”

 

~

 

Red did it again the next night.

 

“uhhhh oh-oh gawwwds! you know you want to -hngh- pump your throbbing. ssslick. member.” Red growled harsh and low, popping the ‘p’ sounds with heavy emphasis.

 

~

 

Then the next.

 

“mm hah, fu-fuhhhck... nuhh, d-do it you freak! heheheh, i’ll bet you're hard enough to shoot c-uhhh-mmm into your own face, huh swap?” his voice was so low and faint that Swap could barely hear him but against his better judgment he was hanging on his every last word. “i bet you're picturing, uh hah hmmm, fucking me as you're doing it too...”

 

Swap felt like he was going to lose it. The glowing tent in his cargo shorts rubbed painfully on his dick, but he didn't want to risk getting any closer to losing than he already was. He wasted no time after that session, sneaking to the bathroom to splash cold water over his face and pumped some liquid hand soap to try washing out the lewd thoughts for good measure but rather than building up suds, his palm bones slipped around his face with an abnormal slickness and a plasticky smell permeated through his nasal aperture. Red had replaced the handsoap with lube at some point. “ugh, you gotta be kidding me...” 

 

Swap ended up lighting that cigarette after all, leaning forward into the Snowdin night from his open bedroom window and flicking ashes into the wind.

 

~

 

Nighttime had became a ritual he anticipated with gritting teeth and sweating through the quick torrent of magic coursing through Swap’s marrow, Red's moans and dirty words working on his sanity. Even his naps had him dreaming fantasies that took a lascivious turn, a stream of consciousness movie of pent up frustration.

 

Punching and kicking until they were snarling, biting Red’s clavicle and neck, dominating a rough kiss to muffle those goddamn moans until he was fumbling with his shorts.

 

Fucking Red in the mouth until his throat was clenching and choking on his dick, making him swallow his cum until it was spilling out the corners of his mouth with his drool.

 

Red writhing underneath him with femurs pushed up to his chest and forcing him to engulf him as deeply as he can go, whimpering and begging for release. 

 

He woke up at his guard post, magic lighting his eye and a primal growl locked behind his teeth. At this rate, he was bound to be driven straight into a early heat. Fortunately, he had an idea to end this damnable competition once and for all.

 

~

 

“it occurs to me, we didn't make a losing condition for you, did we?” Swap mentioned with feigned casualness as Red passed by his station. 

 

“yeah? What about it?” Red smirked. “you want to put a limit on how fast i cum or something?”

 

“nah. pop off as quickly as you want.” He got up and stretched, popping joints through his spine and holding the ball and socket joint of his right shoulder as he rolled it in place. He walked over to stand too close to Red, enjoying the look of suspicion and his not so subtle step backwards. He crowded his space again, keeping his expression relaxed and open as he'd backed Red until the rough bark of a pine tree was grazing Red’s skull, effectively cornering him. “but since i can't jerk off at all at night thanks to this, i think you should have to worry about not getting caught.”

 

“pft, that's easy, boss is a heavy sleeper so i haven't woken him up once yet.” Red said dismissively, even with his eyes locked on Swap's, sweat dripping past his temple. 

 

Swap situated a knee between his legs and pressed close, using his height to lean over him with both hands pressed on each side of Red’s face. “i meant right now. if i make you scream loud enough that Blue comes investigating, that means i win.”

 

“this sounds like revenge for a few sleepless nights,” Red chuckled, his eyes narrowing at the wisp of tangerine magic lighting the taller skeleton's sockets with a rising intensity. “i doubt you'll last a minute in this state.”

 

Swap pressed close, lifting Red's head by the chin to place a deceptively chaste kiss to his grin and murmur into his mouth, “that's what you think.”

 

Red's legs suddenly pulled up, threatening to drop him to the snowy ground, only for his arms to yank in his shoulder joints as they were suspended above his head. “wha-?!”

 

A trailing warmth wound up between his tibia and fibula and tightened over his patella to spread his legs up and out. Swap's magic formed tentacles that kept him supported by his arms pinned further up onto the tree, but also restrained his legs. Red rocked his hips experimentally, to test the constraints of his position, and then leered up to Swap. Swap's eyes were now both aflame with magic, with only a mild smile playing on his face. 

 

Maybe it was only sheer stubbornness in the face of Swap’s devouring gaze, but Red growled challengingly, “don't think you're gonna top.”

 

The tentacles wrapped around his legs began traveling again, leaving a slick heated citrine ooze that cooled in the air, and disappeared into Red's athletic shorts. Red's leer gave way to a halfway embarrassed flush and he yanked hard on his restrained hands to pull himself away from the tentacles. Swap took considerable satisfaction in feeling his magic reach his goal, squeezing through his pelvic inlet and wrapping dexterously through the gaps of his ischium. 

 

“if you're going to do it, stop me. at this point, you're just hanging around. nyeh heh heh” Swap chuckled at his joke, and Red nearly laughed too if it weren't for the sudden tightening of yet another tentacle around his spine. 

 

“heh, you must be really desperate if your magic is doing that.” Red licked his teeth, lapping briefly across the gold tooth. He kicked out into the restraints, only for the tentacles to tighten across his joints and force his legs up close to his chest where the tentacles wrapped around his femurs were joined by slimmer tentacles branching out from the one around his spine and grasping out from between his ribs like vines. His bones were laced tightly together moments later.

 

Swap pulled Red’s shirt up over his ribcage and hungrily watched him squirm with the spinal tentacle winding through the spinal processes. “do it. take control. but i don't think you will.” 

 

The tentacle hugged close, the ectoplasm yielding and swelling around the vertebrae, and then suddenly stroked down Red's spine to the last lumbar vertebrae, leaving a wet slick trail and an electrical current singing through his sensitive bones. Red choked back his cry, jerking his head against the tree with a involuntary flush. “hngh!” 

 

Swap cocked his head, studying his face. “that didn't sound very dominating, red. and more to the point...” He trailed phalanges past the writhing tentacles nested intimately in Red's pelvis to pull the tenting fabric over his formed member, bobbing and pulsing even in the cold air. “you're already rock hard.”

 

“fuck you!” Red spat, his grin strained. A tentacle was wrapping across his dick and tightening around the base of it, while the tip of it probed at the slit. “hrk- nnnuh...”

 

Swap palmed himself, enjoying himself for once since Red’s damned teasing began. His dick was twitching at the sight of Red's face lit up in a heavy flush, framed by his knees, tentacles coaxing out precum from the head of his cock. “that's the idea, red.” 

 

His hips thrusting forward of their own accord, Red’s head lolled forward and he panted. “You really ar-nghh, fuckin’... kind of a freak, aren’t you? ah, hahhhhh…s-shit...”

 

His dick was weeping magic, magic sparking and coiling white hot through the bones underneath the slick tentacles magic. 

 

“HEY, ARE YOU GUYS OKAY?” Blue's voice sounded from far off. They froze, even the tentacles stopping in their steady trails all over Red's body. Swap looked to Red expectantly, eye ridges quirked up mockingly inquisitive as he mouthed:

 

'are you okay?'

 

“y-yeah, we're doing fine blue!! just fucking around again with your asshole brother.”

 

“RED! DO I HAVE TO REMIND YOU THAT CHILDREN PASS BY HERE SOMETIMES?! AND WATCH YOUR LANGUAGE!!!” Blue scolded. Luckily, the sounds of crunching ice under Blue's boots faded and Red sighed. Swap gave him a cheeky grin and placed a finger to his teeth as if he really needed reminding.

 

One tentacle slid up Red's mandible leaving a slimy trail before pressing cautiously around the edges of the delicate bones of his zygomatic process. It traced inward to the socket and convulsively Red gasped and thrust hard into the tentacle stimulating the weeping slit of his member. “oh oh fuck! Hah!”

 

Having confirmed that Red had indeed found that pleasurable, Swap decided to take things further. He unzipped his cargo shorts and pulled his member out, slowly pumping himself. He angled his dick to Red's face and Red automatically moved against the tentacle to wrap his mouth around it, only to realize that he was too low to do more than lick up the base of his shaft. He asked thickly around the drool gathering past his jaw, “huhnn..? w-what are you doing..?”

 

The tentacle prodding the inside of his eye socket drifted down and lodged itself at the hinge of his jaw, forcing his mouth open. Swap murmured, wiping off a little of the residue the tentacle left with his thumb, “i told you, i'm going to make you scream.”

 

He pushed the head of his cock into Red's eyesocket, and pressed in slowly. Red began babbling incoherently at the pain-pleasure burning deep in his marrow and his perception fizzling to make room for the intrusion, “fuck fuck fuuuck ohhhh fuhhhh-” until Swap was halfway sheathed by the incorporeal magic that lay inside. 

 

Red was trembling and jerking into the writhing Swap's tentacles, his dick twitching, and dripping precum to the snow. Once Swap started thrusting shallowly, hands gripping the back of his skull, Red came hard, spurting dual streams of semen over the tentacle that stimulated the slit.

 

“nnngh ohhhh fu-huh-huhhhck..!” Red stuttered, his eye socket continually being thrust into during the course of his orgasm. He finally slouched limply, still held up by the magic, and drained of energy. 

 

Swap grabbed Red underneath his femurs, his magic still present and restraining though the tentacles that'd curled into his pelvis were withdrawing so that Red's entrance could finally form. With no more patience left to continue teasing, Swap sunk into Red, feeling his head spin at the shock waves of toe curling pleasure. He bit Red's clavicle to keep himself quiet, grunting heavily as he buried himself deeper, bottoming out easily in Red’s aroused magic.

 

“so… hot… the way you’re squeezing around me…” Swap, murmured dazedly. He pulled out languidly, before sliding back in a quick hard thrust, and began setting a frantic pace into Red’s yielding body.

 

The tentacle had tightened around the base of Red’s dick as Swap felt his orgasm approaching, and the magic's over-sensitivity had Red thrusting hard against it helplessly. Red was practically sobbing as his dick twitched into hardness again and his second orgasm mounted but held tight in a dizzy delirium.

 

Red was shrieking now, his voice rising unnaturally in pitch, “w-wait t-too sensitive you've still got that tentacle wrapped a-around meEE!!”

 

His muscles was squeezing hard around Swap's dick in response to the overstimulation and Swap thrusted a few more times growling into Red's clavicle. He felt himself being milked by his hot slick heat, losing himself to it until moments later his hips began stuttering and he came so hard it seemed like his vision went white briefly.

 

Still leaning over Red, panting and slowly coming to, he realized that Red was staring up into his eyes and begging, “P-P-PLEASE PLEASE PLEASE I CAN'T C-COME, P-PLEASE LET ME COME..!”

 

Swap released his grip through his magic, stroking Red’s member with his hand to help him along more gently, but the second his tentacle loosened across the base of Red's member he was coming again, shrieking hoarsely, “AHH, THANK YOU THANKYOUUU..!”

 

Red nearly collapsed in the snow with Swap’s magic dissipating, but for Swap shifting their position so he was sitting instead. He couldn't have been bothered to move to someplace more comfortable or even less open to discovery, sweating hard and his joints feeling so wobbly from such a huge release of pent up magic.

 

They nearly nodded off to sleep, except for Red remaining conscious and aware long enough to pull Swap through his shortcut to the house, landing onto the guest mattress that was laid onto the floor in the same position as before. Red shook him a little, shifting his sore hips uncomfortably and feeling his joints pop, then grumbled “hey asshole, i won this round. no one walked in on us, and considering you buried your dick in my ass, i'm pretty sure that more than counts for losing this prank war.”

 

Swap's soft snores met this declaration.

 

“i'm glad you agree.” Red muttered and passed out, too tired to bother moving anymore.

 

~

 

“I TELL THEM THAT CHILDREN PASS BY HERE, SO WHAT DO THEY DO BUT KEEP GOING?!” Blue vented with exasperation, throwing his hands up. “IT’S BAD ENOUGH THEY WERE SEXTING AT NIGHT WHEN YOU’RE TRYING TO SLEEP! THEY’RE LUCKY WE’RE SUCH COOL UNDERSTANDING GUYS THAT WILL DO SLEEPOVERS FOR THEIR PRIVACY, BUT THIS IS RIDICULOUS!!”

 

“MORE IMPORTANTLY, THEY'RE DOING A TERRIBLE JOB AT DATING... THEY SKIPPED NEARLY EVERY STEP IN THE HANDBOOK!” Fell filed at the tips of his phalanges, a habit that never quite went away even after moving into the Swapverse, and idly paged through a dating manual from the Snowdin Librarby. “CLEARLY MY BROTHER NEEDS MORE TRAINING.”

 

Blue turned on his heels and crossed his arms, “I KNOW!!! WHAT THE HECK!! CLEARLY THIS IS THE WORST WAY TO END A DATE.”

  
Fell hummed in agreement. “WE’LL KEEP THEM ON THE STRAIGHT AND NARROW NEXT TIME.”

**Author's Note:**

> So, this ended up being even longer than expected... and then longer! Crazy! It seems to be the theme for everything I've been writing lately. Anyways, if you want to talk to me, just leave a comment or maybe come visit me on tumblr!  
> https://agraulisvanillae.tumblr.com/  
> If you'd like to visit Leedlepie, their tumblr is here!  
> http://leedlepie.tumblr.com/


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